Many vehicles are used over a wide range of vehicle speeds, including both forward and reverse movement. Some types of engines, however, are capable of operating efficiently only within a narrow range of speeds. Consequently, transmissions capable of efficiently transmitting power at a variety of speed ratios are frequently employed. When the vehicle is at low speed, the transmission is usually operated at a high speed ratio such that it multiplies the engine torque for improved acceleration. At high vehicle speed, operating the transmission at a low speed ratio permits an engine speed associated with quiet, fuel efficient cruising.
The transmission speed ratio may be controlled by adjusting the pressure of hydraulic fluid supplied to various clutches. During a shift between speed ratios, a controller may reduce the hydraulic pressure to an off-going clutch and, in a coordinated manner, increase the hydraulic pressure to an on-coming clutch. If the pressures are not carefully coordinated, the shift feels rough to vehicle occupants. A number of noise factors such as environmental conditions, component wear, and part-to-part variation influence the transmission response to controller signals. In order to consistently achieve excellent shift quality, the controller may rely closed loop control using one or measurements of transmission response as feedback signals during a shift. Measurement of transmission output torque is particularly useful.